1. Field of the Invention
The present invention relates to an anode composition for a lithium secondary battery and a lithium secondary battery using the same. More particularly, the present invention relates to an anode composition for a lithium secondary battery that includes a specific conductive material and an anode active material with a shell containing a specific material to ensure an improved life characteristic, and a lithium secondary battery having the same.
2. Description of the Related Art
Various kinds of electrolytes are used for the recently widely used electrochemical devices, lithium secondary batteries, electrolyte condensers, electric double-layered capacitors, and electrochromic display devices, as well as the variously studied dye-sensitized solar cells for future commercialization. The importance of electrolytes is increasing day by day.
In particular, lithium secondary batteries are attracting the most attention for its high energy density and long cycle life. Generally, a lithium secondary battery includes an anode made of carbon material or lithium metal alloy, a cathode made of lithium metal oxide, and electrolyte made by dissolving a lithium salt in an organic solvent.
Initially, lithium metal was used for the anode of a lithium secondary battery. However, due to lithium having a problem of low reversibility and low safety, generally carbon materials are now being used as an anode active material of a lithium secondary battery. The carbon material compared with lithium has low capacity but is advantageous in that it has excellent reversibility and low price.
However, as the use of lithium secondary batteries are increasing, so does the demand for high-capacity lithium secondary batteries. Accordingly, there is a demand for a high-capacity anode active material that may substitute the carbon material having low capacity. In order to meet the demand, attempts were made to use metals, for example Si and Sn, that have a higher charge/discharge capacity than the carbonaceous materials and that allow electrochemical alloying with lithium.
However, such metal-based anode active materials have serious changes in volume, accompanied with charging/discharging of lithium, resulting in the metal-based anode active materials to crack and pulverize. Thus, when charging/discharging cycles are repeated, the metal-based anode active material shows a sudden deterioration of capacity and a shorter cycle life.
In order to solve the above problem, attempts were made to use compounds of metals such as Si and Sn, in other words their oxides or alloys, as an anode active material. However, if the oxide or alloy of metal is used, problems like the pulverization of anode active material and the continuous side reactions with electrolyte are still not solved, even though the life characteristic and the volume expansion prevention are improved in comparison to the case using only metal as an anode active material. Thus, the above method is not a fundamental solution for the above problems.
A new attempt has been made to improve the life characteristic by coating a metal-based anode active material with carbon by means of chemical vapor deposition or thermal treatment such as carbonization. However, these methods are accompanied with high temperature and in the case of the metal-based anode active material, there is a possibility that the structural features or electrochemical properties of the original material may change in accordance with the temperature at which the thermal treatment is performed.
In addition, the degree of dispersion of the conductive material and the anode active material are a great contribution to improving the performance of a battery, when an anode composition is made. Commonly, the conductive material uses spherical conductive carbon fine particles having an average diameter of 30 to 100 nm and a specific surface area of about 1,400 m2/g, but due to its small size and its great specific surface area, this conductive material has difficulty dispersing.